Lindsay Lohan Blasted for 'Heavy Partying'

Lindsay Lohan has been chastised for calling in sick to the set of her movie Georgia Rule – by the head of the company producing the film.

In a stern letter that threatens legal action against the star, James G. Robinson, CEO of Morgan Creek Productions, calls Lohan's recent absences from the set "irresponsible and unprofessional."

The letter, dated July 26 and marked for hand delivery and e-mail, was first posted on The Smoking Gun Web site. A rep for Morgan Creek confirmed the letter's authenticity to PEOPLE.

Early Wednesday morning, Lohan was briefly hospitalized for being "overheated and dehydrated," her rep, Leslie Sloane-Zelnick, told TV's The Insider.

Several sources told PEOPLE that Lohan had been at the nightclub Guy's Tuesday night prior to her hospitalization.

Lohan's rep had no comment. A source close to the actress was surprised by the letter, telling PEOPLE, "She hasn't been crazy partying. In fact, I've never seen her so together."

A publicist for Morgan Creek Productions told PEOPLE on Thursday, "Lindsay reported to the set today on time. We're hoping she remains on schedule for the rest of production."

Below is a transcript of the letter.

Dear Lindsay,

Since the commencement of principal photography of Georgia Rule, you have frequently failed to arrive on time to the set. Today, you did not show for work (all day). I am now told you don't plan to work tomorrow because you are "not feeling well." You and your representatives have told us that your various late arrivals and absences from the set have been the result of illness; today we were told it was "heat exhaustion." We are well aware that your ongoing all night heavy partying is the real reason for your so called "exhaustion." We refuse to accept bogus excuses for your behavior.

To date, your actions on Georgia Rule have been discourteous, irresponsible and unprofessional. You have acted like a spoiled child and in so doing have alienated many of your co-workers and endangered the quality of this picture. Moreover, your actions have resulted in hundreds of thousands of dollars in damage. We will not tolerate these actions any further.

If you do not honor your production commitments, including your scheduled call time for tomorrow, and any call times thereafter, we will hold you personally accountable. This means that in addition to pursuing full monetary damages, we will take such other action as we deem necessary to preserve the integrity of the Georgia Rule Production as well as Morgan Creek's financial interests. I urge you to take this letter seriously and conduct yourself professionally.